The objective of the invention is to provide an exercise instrument capable of providing variable resistance suitable to the exerciser's need.
More specifically, an object of the invention is to provide an exercise instrument of compression/expansion action including tubular casing construction surrounding a hollow plunger unit with exchangeable rubber stoppers of variable central exhaust holes.
A futher object is to provide an exercise instrument whose resistance force increases proportionally in correspondence with the user's input force from almost zero pounds to several hundreds pounds. The variable resistant property of the exercise air pump makes it an ideal exercise instrument for both the beginning and advanced exercisers.
Other features and advantages of the invention will become apparent during the course of the following description.